This invention relates to a magnetic recording medium, such as a hard disk, a flexible disk, a magnetic tape, for use in a magnetic recording device and a method of manufacturing and evaluating the same.
In general, a magnetic recording device of the type described includes a magnetic recording medium, such as a hard disk, and a head slider movable along a surface of the magnetic recording medium. In such a magnetic recording device, a contact start stop (CSS) method is adopted such that the head slider is initially brought into contact with the surface of the magnetic recording medium and is thereafter caused to gradually float off the surface of the magnetic recording medium with a floating gap left between the surface of the magnetic recording medium and the head slider when the magnetic recording device is driven. Thereafter, the head slider is contacted with the surface of the magnetic recording medium and stopped in the CSS method.
A high recording density is recently required for the magnetic recording device so as to store a great deal of information in the magnetic recording device. Herein, it is to be noted that such a high recording density tends to narrow the floating gap between the head slider and the magnetic recording medium more and more. As long as the CSS method is adopted in the magnetic recording device, the head slider is slid along the magnetic recording medium and should be inevitably brought into contact with the magnetic recording medium intermittently or periodically.
With this structure, contacting the magnetic recording medium with the head slider gives rise to various kinds of tribological problems which bring about serious degradation of reliability of the magnetic recording device. Especially, a head crash phenomenon becomes very serious which stems from friction on an interface between the head slider and the magnetic recording medium during a sliding and contacting state. Such an interface may be called a head disk interface (will be abbreviated to HDI hereinunder). This is because such friction results in a mechanical and frictional damage on the magnetic recording medium and a final destruction of a magnetic layer included in the magnetic recording medium.
In order to solve the tribological problems, a wide variety of proposals have been offered as will be described hereinunder.
For example, a magnetic recording device which is disclosed in Japanese Unexamined Patent Publication No. 4-109427, namely, 109427/1992 includes a protection film of metal oxide, nitride, or the like as an outermost layer. Specifically, the magnetic recording medium comprises a nonmagnetic substrate formed by a combination of a nonmagnetic base and an underlying layer, an intermediate layer on the nonmagnetic substrate, and a magnetic layer on the intermediate layer. The protection film is overcoated or overlaid on the magnetic layer to prevent the head slider from being directly contacted with the magnetic layer. With this structure, it is also possible to improve friction resistance and abrasion resistance.
Alternatively, a lubricant layer is formed by a lubricant coated on the protection film so as to furthermore improve the abrasion resistance. In addition, the lubricant layer is also effective to improve lubricity of the magnetic recording medium.
According to the inventor's experimental studies, it has been found out that the lubricant is very often composed of a compound of a fluorocarbon system, a hydrocarbon system, or the like and includes a polymer which is formed by a union of simple molecules concatenated in chain and which has polar groups at two ends of the chain. Furthermore, it has been found out that, when such a lubricant is coated on the protection layer, the tribological property at the HDI is mainly determined by a coating amount of the lubricant.
More specifically, when the coating amount of the lubricant is insufficient, the protection layer is directed brought into contact with the head slider which is being slid along the magnetic recording medium. As a result, a damage is liable to occur on the magnetic recording medium or the head slider.
On the other hand, when an excessive amount of the lubricant is coated on the protection layer, the head slider is apt to be stuck to the magnetic recording medium and, as a result, stiction-often takes place between the head slider and the magnetic recording medium and makes rotation of the magnetic recording device difficult. This deteriorates a function of the lubricant layer.
From this fact, it is inferred that a remnant amount of a lubricant serves to determine both a distribution state of the lubricant on the protection layer and lublicity of the lubricant layer at the HDI. Therefore, it is preferable that the remnant amount of the lubricant is accurately measured with the lubricant layer coated on the protection layer.
Practically, the remnant amount of the lubricant might be measured either by an ellipsometric method or by the use of a Fourier transform-infrared spectrophotometer (FT-IR), as suggested in the articles disclosed in "Tribology and Mechanics of Magnetic Storage Systems" (Vol.5, SP-25, STLE, 1988, pages 55-61) and IEEE Trans. Magn., Vol. 26, No.5 September, 1990, pages 2688-2690.
With the above-mentioned methods, no consideration is made at all in connection with the case where a lubricant film is dispersed on the protection film in the form of lubricant film islands with the protection film partially uncovered. Specifically, an average thickness can not be precisely determined by the above-mentioned methods because it is difficult to accurately calculate the average thickness of the lubricant film islands which are dispersed with uncovered portions partially left on the protection film.
Accordingly, the above-mentioned methods can not accurately evaluate the lubricant film which is dispersed in the form of the lubricant film islands. As a result, a uniform lubricant layer can not be coated on the protection film with these methods. This shows that an invariable characteristic can not be insured.
In Japanese Unexamined Patent Publication No. Hei 4-168622, a proposal has been made regarding a method of pre-processing the protection film by a laser beam and plasma before the lubricant layer is coated. With this method, the protection film is divided by the pre-processing into two regions which has good and bad wettability for the lubricant. The lubricant is deposited only on the region having the good wettability and serves to selectively form the lubricant films on the protection film.
However, no teaching is made in the above patent publication neither about a method of evaluating the lubricant film or about an optimum lubricant film. Thus, it is impossible to know conditions of the optimum lubricant film from the patent publication.